1. Field of the Invention
The invention disclosed herein relates to oil field exploration and, in particular, to detection of friction between instrumentation downhole and the surrounding environment.
2. Description of the Related Art
One of the most severe problems that can occur when drilling a hole into the ground, for example a hydrocarbon exploration well, is the inability to remove the drill string from the borehole. There are many possible reasons for such an event. Two very common reasons are insufficient hole cleaning and swelling formation. When the mud circulation is inappropriate, it is not capable of carrying all cuttings to surface. Over time, the cuttings accumulate in the annulus between the drill string and the borehole wall. Increasing friction between the drill string and the cuttings eventually exceeds the available torque and pull force, and the string becomes stuck. Some formations will slowly decrease the borehole diameter (e.g. due to reactions with the drilling mud or due to insufficient strength). The reduced borehole diameter increases the friction acting upon the drill string, in some cases up to a point where the torque and pulling capacity of the drilling rig is exceeded, and the string becomes stuck.
In the prior art approaches were taken to address stuck strings. As an example, some solutions tried to predict such events by monitoring the circulating pressure, the drilling torque or the vibration characteristics of the drill string or the Bottom Hole Assembly (BHA). The drilling torque and the changing vibration characteristics are effects caused by increasing friction. Measuring the friction itself provides a more direct knowledge of the situation, facilitating the prevention of a stuck pipe event.
Therefore, what are needed are methods and apparatus that help to prevent stuck pipe resulting from poor hole cleaning or swelling formation. Preferably, the methods and apparatus provide for measuring frictional forces in play on an exterior surface of the pipe.